But wait... If they're the "smiley" type of religious people, then I suppose there won't be drama - at least not on the surface. But there might be resentment underneath or dh might fear there will be. Imo, dh's feelings are key here.
But Henanbien, I'm not sure what you mean when you say they "take over" the kitchen and bathroom. Do you mean with their things? Or that you can't get in there when you want?
Have you tried to do anything about this? If it's a matter of their spreading out, could you give them a shelf in the bathroom or a space on the kitchen counter or something specifically for their things? Would that help? If it's that you can't get in when you want, have you tried speaking up about it? "I need to get in the bathroom to take my medicine."... "I'm going to be using the kitchen between 4 & 6" or whatever.
Also, given your back problems, I hope dh does most of the entertaining and cleaning up when they're there. In fact, I hope they clean up after themselves and maybe cook a meal or two for you people or take you out to dinner once or twice.